Method and apparatus for capture verification and threshold determination

ABSTRACT

An apparatus and method for verifying capture by a pacing pulse in which a test depolarization waveform recorded during a pacing event is compared with a template waveform representing capture by the pacing pulse. Capture verification in this manner may be used in pacemakers having multiple pacing channels for the atrial and/or ventricles where the multiple paces can interfere with conventional sensing of evoked responses in order to verify capture.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a divisional of U.S. patent application Ser. No.10/003,718, filed on Oct. 26, 2001, now U.S. Pat. No. 7,177,689, thespecification of which is incorporated herein by reference.

FIELD OF THE INVENTION

This invention pertains to cardiac pacemakers and, in particular, tosystems and methods for ascertaining the performance of the device andadjusting pacing parameters accordingly.

BACKGROUND

Implantable cardiac pacemakers are a class of cardiac rhythm managementdevices that provide electrical stimulation in the form of pacing pulsesto selected chambers of the heart. (As the term is used herein, apacemaker is any cardiac rhythm management device with a pacingfunctionality regardless of any additional functions it may perform suchas cardioversion/defibrillation.) Pacemakers typically have aprogrammable electronic controller that causes the pacing pulses to beoutput in response to lapsed time intervals and sensed electricalactivity (i.e., intrinsic heart beats). Most pacemakers are programmedto operate in a so-called demand mode (a.k.a., synchronous mode), wherea pacing pulse is delivered to a heart chamber during a cardiac cycleonly when no intrinsic beat by the chamber is detected. An escapeinterval is defined for each paced chamber, which is the minimum timeinterval in which a beat must be detected before a pace will bedelivered. The ventricular escape interval thus defines the minimum rateat which the pacemaker will allow the heart to beat, sometimes referredto as the lower rate limit. If functioning properly, the pacemaker inthis manner makes up for a heart's inability to pace itself at anappropriate rhythm.

In order for a pacemaker to control the heart rate in the mannerdescribed above, the paces delivered by the device must achieve“capture,” which refers to causing sufficient depolarization of themyocardium that a propagating wave of excitation and contraction result(i.e., a heart beat). A pacing pulse that does not capture the heart isthus an ineffective pulse. This not only wastes energy from the limitedenergy resources (battery) of pacemaker, but can have deleteriousphysiological effects as well, since a demand pacemaker that is notachieving capture is not performing its function in enforcing a minimumheart rate. A number of factors can determine whether a given pacingpulse will achieve capture, but the principal factor of concern here isthe energy of the pulse, which is a function of the pulse's amplitudeand duration. The minimum pacing pulse energy necessary to achievecapture by a particular pacing channel is referred to as the capturethreshold. Programmable pacemakers enable the amplitude and pulse widthof pacing pulses to be adjusted, along with other parameters. It iscommon practice to determine the capture threshold by initially pacingwith a high energy to ensure capture and then progressively lowering thepacing pulse energy during a sequence of cardiac cycles until capture isno longer achieved. The pacing pulse energy can then be adjusted to anappropriate value in accordance with the determined capture threshold bysetting it equal to the capture threshold plus a specified safetymargin.

A common technique used to determine if capture is present during agiven cardiac cycle is to look for an “evoked response” immediatelyfollowing a pacing pulse. The evoked response is the wave ofdepolarization that results from the pacing pulse and evidences that thepaced chamber has responded appropriately and contracted. By detectingan evoked atrial or ventricular depolarization that exceeds a specifiedvalue (i.e., corresponding to an evoked P-wave or evoked R-wave,respectively, on a surface electrocardiogram or their equivalents in aninternal electrogram), the pacemaker is able to detect whether thepacing pulse (A-pulse or V-pulse) was effective in capturing the heart,that is, causing a contraction in the respective heart chamber. Captureverification can be performed in the clinical setting, with theclinician then adjusting pacing parameters so that the heart is reliablypaced. It is desirable, however, for the pacemaker itself to be capableof verifying capture so that loss of capture can be detected when itoccurs with pacing parameters then adjusted automatically, a functionknown as autocapture. (See, e.g., U.S. Pat. No. 6,169,921 issued toKenKnight, et. al. and presently assigned to the Guidant Corp.) Anautocapture function provides the pacemaker with extended longevity,greater ease of use, and greater patient safety.

Also included within the concept of cardiac rhythm is the manner anddegree to which the heart chambers contract during a cardiac cycle toresult in the efficient pumping of blood. For example, the heart pumpsmore effectively when the chambers contract in a coordinated manner. Theheart has specialized conduction pathways in both the atria and theventricles that enable the rapid conduction of excitation (i.e.,depolarization) throughout the myocardium. These pathways conductexcitatory impulses in a manner that results in a coordinatedcontraction of both atria and both ventricles. Without thesynchronization afforded by the normally functioning specializedconduction pathways, the heart's pumping efficiency is greatlydiminished. Patients who exhibit pathology of these conduction pathways,such as bundle branch blocks, can thus suffer compromised cardiacoutput. The resulting diminishment in cardiac output may be significantin a patient with congestive heart failure (CHF) whose cardiac output isalready compromised. Intraventricular and/or interventricular conductiondefects can also be a cause of CHF in some patients. In order to treatthese problems, pacemakers have been developed which provide multi-siteelectrical pacing stimulation to one or both of the atria and/orventricles during a cardiac cycle in an attempt to improve thecoordination of atrial and/or ventricular contractions, termed cardiacresynchronization therapy. To optimize the cardiac output for some heartfailure patients, for example, the right and left ventricles are pacedsynchronously with a determined time offset, termed biventricularpacing.

Multi-site resynchronization pacing, however, is problematic forconventional capture verification methods based upon evoked responsedetection as described above. In biventricular pacing, for example, theproximity in time of resynchronization paces to the left and rightventricles may prevent an evoked response caused by the first pace frombeing distinguished from the second pace. In addition, the second pacecould interfere with evoked response sensing when the evoked responsefrom the first pace occurs within an amplifier blanking intervalinitiated by the second pace.

SUMMARY OF THE INVENTION

A depolarization waveform, such as a surface electrocardiogram (ECG) orinternal electrogram from by an implanted pacemaker, recorded during apaced event that achieves capture exhibits morphological differencesfrom that recorded during a paced event that fails to achieve capture.Also, when multiple pacing pulses are delivered to either the atria orthe ventricles during a cardiac cycle, the morphology of thedepolarization waveform that results is affected if even one of thepacing pulses fails to achieve capture. In accordance with theinvention, capture of the heart by a pacing pulse is determined bycomparing a test depolarization waveform recorded during the paced eventwith a template waveform representing capture of the heart by asimilarly delivered pacing pulse. The comparison may be done bycross-correlating the template and test waveforms so that loss of thecapture is detected when the two waveforms become uncorrelated. In amulti-site pacing situation, template waveforms representing capture byeach pace individually and by all of the paces collectively can be useddetermine which pace failed to achieve capture and to simplify thedetermination of capture thresholds for each pacing site.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a multi-site pacemaker.

FIGS. 2A and 2B illustrate an ECG recorded after a pace and a templateECG.

FIGS. 3A and 3B illustrate exemplary embodiments of algorithms forcapture threshold determination.

FIGS. 3C and 3D illustrate exemplary embodiments of an auto-capturealgorithm.

DETAILED DESCRIPTION

The present invention may be incorporated into pacemakers having anumber of different pacing configurations, including multi-site pacingconfigurations for delivering various types of resynchronization therapywhere a pace is delivered to each of the paired atria and/or ventriclesduring a cardiac cycle or where multiple paces are delivered to a singlechamber. For illustrative purposes, however, a the invention will bedescribed with reference to a dual-chamber pacemaker (i.e., one thatsenses and/or paces both the atria and ventricles) having twoventricular pacing channels for pacing both ventricles or delivering twopaces to a single ventricle as shown in FIG. 1.

a. Hardware Platform

Pacemakers are typically implanted subcutaneously on a patient's chestand have leads threaded intravenously into the heart to connect thedevice to electrodes used for sensing and pacing. A programmableelectronic controller causes the pacing pulses to be output in responseto lapsed time intervals and sensed electrical activity (i.e., intrinsicheart beats not as a result of a pacing pulse). Pacemakers senseintrinsic cardiac electrical activity by means of internal electrodesdisposed near the chamber to be sensed. A depolarization wave associatedwith an intrinsic contraction of the atria or ventricles that isdetected by the pacemaker is referred to as an atrial sense orventricular sense, respectively. In order to cause such a contraction inthe absence of an intrinsic beat, a pacing pulse (either an atrial paceor a ventricular pace) with energy above the capture threshold must bedelivered to the chamber.

The controller of the pacemaker is made up of a microprocessor 10communicating with a memory 12, where the memory 12 may comprise a ROM(read-only memory) for program storage and a RAM (random-access memory)for data storage. The controller could be implemented by other types oflogic circuitry (e.g., discrete components or programmable logic arrays)using a state machine type of design, but a microprocessor-based systemis preferable. The controller is capable of operating the pacemaker in anumber of programmed modes where a programmed mode defines how pacingpulses are output in response to sensed events and expiration of timeintervals. A telemetry interface 80 is provided for communicating withan external programmer 300. The external programmer is a computerizeddevice with a controller 330 that can interrogate the pacemaker andreceive stored data as well as adjust the operating parameters of thepacemaker.

The pacemaker has an atrial sensing/pacing channel comprising ringelectrode 33 a, tip electrode 33 b, sense amplifier 31, pulse generator32, and an atrial channel interface 30 which communicatesbidirectionally with a port of microprocessor 10. The device also hastwo ventricular sensing/pacing channels that similarly include ringelectrodes 43 a and 53 a, tip electrodes 43 b and 53 b, sense amplifiers41 and 51, pulse generators 42 and 52, and ventricular channelinterfaces 40 and 50. For each channel, the electrodes are connected tothe pacemaker by a lead and used for both sensing and pacing. A MOSswitching network 70 controlled by the microprocessor is used to switchthe electrodes from the input of a sense amplifier to the output of apulse generator. The pacemaker also has an evoked response sensingchannel that comprises an evoked response channel interface 20 and asense amplifier 21 that has its differential inputs connected to aunipolar electrode 23 and to the device housing or can 60 through theswitching network 70. The evoked response sensing channel may be used toverify that a pacing pulse has achieved capture of the heart in aconventional manner or, as explained below, used to record an evokedresponse electrogram.

The channel interfaces include analog-to-digital converters fordigitizing sensing signal inputs from the sensing amplifiers, registersthat can be written to for adjusting the gain and threshold values ofthe sensing amplifiers, and, in the case of the ventricular and atrialchannel interfaces, registers for controlling the output of pacingpulses and/or adjusting the pacing pulse energy by changing the pulseamplitude or pulse width. The microprocessor 10 controls the overalloperation of the device in accordance with programmed instructionsstored in memory. The sensing circuitry of the pacemaker generatesatrial and ventricular sense signals when voltages sensed by theelectrodes exceed a specified threshold. The controller then interpretssense signals from the sensing channels and controls the delivery ofpaces in accordance with a programmed pacing mode. The sense signalsfrom any of the sensing channels of the pacemaker in FIG. 1 can bedigitized and recorded by the controller to constitute an electrogramthat can either be transmitted via the telemetry link 80 to the externalprogrammer 300 or stored for later transmission. The patient's cardiacactivity may thus be observed in real-time or over a selected historicalperiod.

In accordance with the invention, an electrogram can also be recorded ofan evoked response to a pace and used to determine if capture isachieved by comparing the recorded electrogram with a templateelectrogram representing capture of the heart by a similarly deliveredpace. An evoked response is the wave of depolarization that results froma pacing pulse and, since it evidences that the paced chamber hasresponded appropriately and contracted, it can be used to verify thatthe pace has achieved capture of the heart. An evoked response sensingchannel for recording an electrogram can be a sensing channel normallyused for other purposes or can be a sensing channel dedicated to sensingevoked responses. It is preferable to record the electrogram with aunipolar electrode that “sees” a larger volume of the myocardium as awave of electrical activity spreads than a bipolar electrode. In theembodiment illustrated in FIG. 1, the atrial and ventricular sensingpacing channels utilize bipolar electrodes, and a dedicated evokedresponse sensing channel is provided with a unipolar electrode.Alternate embodiments may employ unipolar electrodes in the atrialand/or sensing/pacing channels, in which case unipolar sensing of anevoked response may be performed with those channels instead of adedicated channel.

b. Capture Verification and Threshold Determination

In accordance with the invention, capture of heart by multiple pacingpulses delivered to the atria and/or ventricles during a cardiac cycleis determined by recording a test depolarization waveform during thepaces and comparing the test waveform with a template depolarizationwaveform representing capture of the heart by at least one pacing pulse.Although the method described herein for capture verification andthreshold determination may be applied to any multi-site pacingconfiguration, the following detailed explanation and description ofspecific embodiments will be confined to a biventricular pacingconfiguration where both ventricles are paced during a cardiac cycleseparated by a programmed offset.

Delivery of multiple paces to the ventricles during a cardiac cyclechanges the pattern of the resulting depolarization as compared with thepattern that results from a single ventricular pace. This differenceappears as a QRS wave morphology change in a recorded depolarizationwaveform such as a surface ECG or electrogram that senses thetime-varying net dipole vector produced by the depolarization. Areference morphology template waveform can be created by recording aventricular ECG or electrogram during a biventricular pacing cycle thatis known to achieve capture with both pacing pulses. Presence or absenceof capture for a given pace can then be determined by comparing thetemplate waveform with a test depolarization waveform recorded duringthe pace. FIG. 2A shows an example of a template ECG waveform TMP and atest ECG waveform TST that match, while FIG. 2B show test and templatewaveforms that are morphologically different because of a failure toachieve capture by one of the pacing pulses. The comparison may beaccomplished, for example, by performing a time-domain cross-correlationbetween the template and test waveforms. Loss of capture at one of theventricular pacing sites is then indicated by a loss of correlationbetween the test and template waveforms. The exact correlation valuesthat should optimally be used in deciding whether or not a test waveformand template waveform match may be selected on the basis of empirictesting as the optimum values may vary for an individual patient and/orpacemaker. Capture verification performed in this manner may be used todetermine the capture threshold of a pacing channel by varying thepacing pulse energy and finding the minimum energy that results incapture.

Capture verification and threshold determination as described above maybe accomplished in a number of different ways. In one exemplaryembodiment, a surface ECG is recorded with conventional leads duringpacing by an external programmer that communicates with the implantedpacemaker via a radio telemetry link. The processor of the externalprogrammer then performs the correlation between the test ECG and atemplate ECG to determine if capture is achieved by the pacing pulses.In a modification to this embodiment, rather than using surface ECGs, atest electrogram recorded by an evoked response sensing channel of thepacemaker and transmitted to the external programmer is compared with atemplate electrogram to verify capture. The external programmer canemploy the telemetry link to adjust the pacing pulse energy in order todetermine the capture threshold and then set the pacing pulse energy atan appropriate value, either under the direction of a clinician orautomatically by software running in the external programmer.

In another embodiment, the controller of the pacemaker is programmed toverify capture by comparing the test electrogram with the templateelectrogram and to determine the capture threshold by varying the pacingpulse energy, either autonomously at selected times or in accordancewith instructions received over the telemetry link. The controller maythen be further programmed to automatically set the pacing pulse energyin accordance with the determined capture threshold. Determination ofthe capture threshold may be performed automatically on a periodic basisor at the direction of a clinician communicating with an externalprogrammer. The controller may also be programmed to verify capture bypacing pulses on a beat-to-beat basis. If a loss of capture is detected,the controller can then perform a capture threshold determination andadjust the pacing pulse energy as appropriate. Loss of capture eventsmay also be logged in the memory of the controller for latertransmission to an external programmer.

FIG. 3A illustrates an exemplary procedure for determining the thresholdvoltage of the right and left ventricular pacing channels (referred toas RV and LV, respectively) in a bi-ventricular pacemaker using ECG orelectrogram waveforms. The auto-threshold algorithm begins at steps A1and A2 by pacing both chambers of the heart and recording an ECG orelectrogram to create a biventricular (Bi-V) template waveform that isto be used as a reference. The pacing pulse amplitude for bothventricles is set at a relatively high value to ensure capture duringacquisition of the biventricular template waveform. After the templatewaveform is obtained, the system decreases one of the pacing amplitudesat step A3, in this case the RV pacing amplitude, before the next pace.The RV pace triggers the recording of an incoming ECG or electrogramfollowing the pace that is to be used as the test waveform in verifyingcapture. A cross correlation analysis is performed between the templatewaveform and the test waveform at step A4. If the waveforms correlatewell, then both ventricular pacing channels are assumed to have achievedcapture and step A3 is repeated to decrease the RV pacing amplitude. Ifloss of correlation is detected at step A4, then the RV pacing amplitudeis assumed to have dropped below the threshold voltage. The capturethreshold is then determined at step A5 to be the RV pacing amplitudebefore the decrease at step A3. The system then sets the RV pacing pulseamplitude to the threshold voltage plus some safety factor at step A6.Steps A3 through A6 are then repeated for the LV pacing channel asindicated by step A7 in order find the LV capture threshold and set theLV pacing amplitude.

In single-site pacing systems utilizing capture verification, it isdesirable to quickly pace the heart once a loss of capture occurs. Thisbecomes especially important with pacing-dependent patients in order tomaintain cardiac activity. Often the delay associated with the externalprogrammer ECG and with the telemetry systems used for communicationbetween the external programmer and the pacemaker can prohibit immediatesafety pacing. Note, however, that the bi-ventricular auto-thresholdalgorithm presented above inherently includes a safety back-up pace withthe additional ventricular pacing channel. Once one channel losescapture, the other still causes contraction of the ventricles,maintaining ventricular function. Because of the safety provided by twoventricular pacing sites, the auto-threshold algorithm could also startwith one output high and increase the other from a sub-thresholdvoltage. For example, a template can be created for RV-only pacing. TheLV pacing amplitude then increases from a sub-threshold voltage untilthe system detects Bi-V pacing. This flexibility thus facilitates theuse of more efficient search algorithms to speed convergence to theproper threshold value.

Another exemplary procedure is illustrated by FIG. 3B that decreases thetotal time of the auto-threshold algorithm by determining the LV and RVcapture thresholds simultaneously. The algorithm first acquirestemplates in the RV-only, LV-only, and Bi-V pacing configurations atsteps B1, B2, and B3. After creation of the templates, the system beginsdecreasing RV and LV pacing amplitudes simultaneously with each pace asindicated by steps B4 and B5, respectively. Similar to the previousalgorithm, the RV pace triggers the creation of a test waveform. Crosscorrelations are then performed between the test waveform and the threetemplates. If a high correlation exists between the test waveform andthe Bi-V template at step B6, both pace amplitudes are assumed to stillbe above the capture threshold value and the algorithm returns to stepsB4 and B5. Otherwise cross-correlations between the test waveform andthe RV-only and LV-only templates are performed at step B7. If a highcorrelation exists between the LV-only template and the test waveform,then the RV pacing amplitude has dropped below the threshold voltage,and the RV capture threshold is found at step B10. Likewise, a highcorrelation between the test waveform and the RV-only template indicatesthat the LV pace amplitude has dropped below the threshold voltage, andthe LV capture threshold is found at step B8. If a capture threshold isfound for a pacing channel at either step B8 or B10, steps B9 and B11then test whether a capture threshold for the other pacing channel hasbeen found so that the procedure can either end at step B13 or return tostep B4 or B5. If the system indicates no correlation between the testwaveform and any of the templates, then both pacing channels havedropped below the threshold value. The capture thresholds for bothpacing channels are then found so that the pacing thresholds can beadjusted accordingly as indicated by steps B12 and B13.

The auto-threshold algorithms illustrated in FIGS. 3A and 3B may beperformed by either the pacemaker controller or the processor of anexternal programmer when it is desired to determine the capturethresholds for the RV and LV pacing channels and set the pacingamplitudes accordingly. As noted above, however, capture verification bycross-correlating template and test waveforms may also be performed on abeat-to-beat basis by the pacemaker controller to provide an ambulatoryauto-capture function. FIGS. 3C and 3D illustrate exemplary algorithmsfor implementing auto-capture in which a capture verification test isperformed with each pace.

Referring first to FIG. 3C, the controller performs an auto-thresholdalgorithm at step C1 in which templates are acquired in the RV-only,LV-only, and Bi-V pacing configurations and capture thresholds aredetermined for the LV and RV pacing channels so that the pacing pulseamplitudes can be set accordingly. The device then operates normallywhile the algorithm waits for a paced beat at step C2. At step C3, anincoming signal is used as a test waveform and cross-correlated with theBi-V template to ascertain if both the RV and LV pacing pulses haveachieved capture. If the Bi-V template and test waveforms are highlycorrelated, capture is assumed, and the algorithm loops back to step C2.If a lack of correlation between the test waveform and the Bi-V templateis found, the algorithm separately cross-correlates the test waveformwith the LV and RV templates at step C4. If the test waveform matchesthe RV template, lack of capture in the LV pacing channel is assumed.The LV pacing pulse amplitude is then increased at step C5, and thealgorithm returns to step C1 so that updated templates can be acquiredand an updated capture threshold determined. Similarly, if the testwaveform matches the LV template, the RV pacing pulse amplitude isincreased at step C6, and the algorithm returns to step C1. If neitherthe RV nor the LV paces have achieved capture as indicated by a lack ofcorrelation between the test waveform and the two templates, both the RVand LV pacing amplitudes are increased at step C7. An auto-thresholdalgorithm is then performed at step C8, with the templates and capturethresholds updated and the pacing pulse amplitudes set accordingly. Acapture verification test is performed at step C9 as the device operateswith the updated pacing pulse amplitudes. If capture is achieved, thealgorithm returns to the capture verification loop of steps C2 and C3.If subsequent paces still fail to achieve capture, it can be assumedthat the lack of capture is due to factors other than pacing pulseenergy such as the occurrence of fusion events, difficulties inobtaining reference templates, or the occurrence of a malfunction in thepacemaker or lead system. An indication that further intervention isrequired is then logged in memory at step C10 which can be communicatedto a clinician during the next communications session with an externalprogrammer.

The ambulatory auto-capture algorithm presented in FIG. 3C relies on theinherent safety of having multiple ventricular pacing sites in theventricle. In the event that one chamber loses capture, there is a lowprobability that the other chamber will simultaneously lose capture.Nonetheless, there is a possibility that the pacemaker could losecapture on both chambers simultaneously. When capture of the ventriclesdoes not occur, it is desirable to provide a back-up safety pace to theright ventricle to immediately provide pacing therapy to prevent thepatient from feeling light headed or loosing consciousness. Dependingupon the particular implementation, the template cross-correlationalgorithms presented here could take greater than 100 ms to accuratelyidentify pacing activity. This is usually too long of a delay to delivera safety pace. Further, if a fusion event occurs, the device mustprevent pacing into a t-wave, so it must again react quickly if a safetypace is to be delivered. FIG. 3D is a flowchart diagram showing anambulatory auto-capture algorithm that uses a traditional evokedresponse comparator in addition to template recognition. Steps C1through C10 in FIG. 3D are identical to those described above withreference to FIG. 3C. After each paced beat, however, the algorithm alsotests for capture at step C11 with an evoked response comparator thatlooks for any evoked response above a specified threshold following apace. If any evoked response occurs from the ventricles, then somecardiac ventricular activity is assumed to have occurred, and thealgorithm proceeds to step C3 to perform the template correlations anddetermine which chamber or chambers were captured. If no evoked responseoccurs following a pace, on the other hand, then the algorithm applies asafety pace to the right ventricle at step C12 and then proceeds as ifneither pacing pulse captured by going to step C7. In this manner, thepatient receives pacing therapy without a noticeable delay.

In the capture verification methods described above, a testdepolarization waveform, such as an electrogram or ECG signal, isrecorded and compared with one or more template waveforms. In certainimplementations, this may involve the processor of the pacemaker orexternal programmer storing samples of a segment of the test waveform inmemory and then performing the cross-correlation operation withcorresponding samples of a template waveform. Recording and correlationof the test waveform with a template, however, may also be implementedby passing samples of the incoming electrogram or ECG signal through afinite impulse filter that performs the cross-correlation operation. Inthat case, the filter may be a matched filter having an impulse responseequal to a time-reversed version of a template waveform. The testwaveform is thus cross-correlated with a template waveform representedby the filter coefficients of the matched filter. Such a matched filtermay be provided for each of the RV-only, LV-only, and Bi-V templatewaveforms and may be implemented either in code executed by thecontroller or as one or more dedicated hardware components.

Capture verification by comparing test and template depolarizationwaveforms of an evoked response has been described above in the contextof multi-site pacing where either one or both of the paired atria or oneor both of the paired ventricles are paced with multiple paces during acardiac cycle. It should also be appreciated that a test depolarizationwaveform, such as an electrogram from an evoked response sensingchannel, can be recorded during delivery of a single pacing pulse andthen compared with a template waveform representing single-site captureof the heart by a pacing pulse in order to determine if capture has beenachieved by the delivered pacing pulse.

Although the invention has been described in conjunction with theforegoing specific embodiment, many alternatives, variations, andmodifications will be apparent to those of ordinary skill in the art.Such alternatives, variations, and modifications are intended to fallwithin the scope of the following appended claims.

1. A cardiac pacemaker, comprising: pulse generators for incorporatinginto first and second pacing channels; a controller for controlling theoperation of the pulse generators and programmed to deliver pacingpulses to either both of the ventricles or both of the atria through thefirst and second pacing channels in accordance with a programmed pacingmode; a sense amplifier for incorporating into an evoked responsesensing channel for sensing an evoked response generated after a pacingpulse; and, wherein the controller is programmed to: deliver first andsecond pacing pulses from the first and second pacing channels,respectively, during a cardiac cycle; record a test electrogram from theevoked response sensing channel during delivery of the first and secondpacing pulses; compare the test electrogram with template electrogramsrepresenting the following capture events: capture of the heart by bothof the first and second pacing pulses, capture by a pacing pulsedelivered only from the first pacing channel, and capture by a pacingpulse delivered only from the second pacing channel; determine whetheror not each of the first and second pacing pulses have achieved capturein accordance with which of the template electrograms is most similar tothe test electrogram.
 2. The pacemaker of claim 1 wherein the controlleris programmed to log a loss of capture event in memory.
 3. The pacemakerof claim 1 wherein the controller is programmed to compare the test andtemplate electrograms by performing a time-domain cross-correlation. 4.The pacemaker of claim 1 wherein the controller is further programmed tovary the pulse energy of the pacing pulses in order to determine acapture threshold for a pacing channel.
 5. The pacemaker of claim 4wherein the controller is further programmed to lower the pacing pulseenergy of a pacing channel until capture is no longer achieved by thatchannel in order to determine the capture threshold.
 6. The pacemaker ofclaim 5 wherein the controller is programmed to determine a capturethreshold for each of the first and second pacing channels by loweringthe pacing energy of each pacing channel separately until the testelectrogram no longer matches the template electrogram representingcapture by both of the first and second pacing pulses.
 7. The pacemakerof claim 5 wherein the controller is programmed to determine a capturethreshold of the first and second pacing channels by: lowering thepacing energy of the first pacing channel until the test electrogrammatches the template electrogram representing capture by the secondpacing pulse but does not match the template electrogram representingcapture by the first pacing pulse; and, lowering the pacing energy ofthe second pacing channel until the test electrogram matches a templateelectrogram representing capture by the first pacing pulse but does notmatch the template electrogram representing capture by the second pacingpulse.
 8. The pacemaker of claim 4 wherein the controller is furtherprogrammed to adjust the pacing pulse energy of a pacing channel inaccordance with the results of the capture threshold determination. 9.The pacemaker of claim 1 wherein the controller is programmed todetermine if capture has been achieved during each cardiac cycle on abeat-to-beat basis.
 10. The pacemaker of claim 1 wherein the controlleris further programmed to deliver a safety pace if capture has not beenachieved during a cardiac cycle.
 11. A method comprising: deliveringfirst and second pacing pulses from first and second pacing channels,respectively, during a cardiac cycle, wherein the first and secondpacing pulses are delivered to either the right and left ventricles orthe right and left atria; recording a test electrogram from an evokedresponse sensing channel during delivery of the first and second pacingpulses; comparing the test electrogram with template electrogramsrepresenting the following capture events: capture of the heart by bothof the first and second pacing pulses, capture by a pacing pulsedelivered only from the first pacing channel, and capture by a pacingpulse delivered only from the second pacing channel; determining whetheror not each of the first and second pacing pulses have achieved capturein accordance with which of the template electrograms is most similar tothe test electrogram.
 12. The method of claim 11 further comprisinglogging a loss of capture event in memory.
 13. The method of claim 11further comprising varying the pulse energy of the pacing pulses inorder to determine a capture threshold for a pacing channel.
 14. Themethod of claim 13 further comprising determining a capture thresholdfor each of the first and second pacing channels by lowering the pacingenergy of each pacing channel separately until the test electrogram nolonger matches the template electrogram representing capture by both ofthe first and second pacing pulses.
 15. The method of claim 13 furthercomprising determining a capture threshold of the first and secondpacing channels by: lowering the pacing energy of the first pacingchannel until the test electrogram matches the template electrogramrepresenting capture by the second pacing pulse but does not match thetemplate electrogram representing capture by the first pacing pulse;and, lowering the pacing energy of the second pacing channel until thetest electrogram matches a template electrogram representing capture bythe first pacing pulse but does not match the template electrogramrepresenting capture by the second pacing pulse.
 16. A systemcomprising: means for delivering first and second pacing pulses fromfirst and second pacing channels, respectively, during a cardiac cycle;means for recording a test electrogram from an evoked response sensingchannel during delivery of the first and second pacing pulses; means forcomparing the test electrogram with template electrograms representingthe following capture events: capture of the heart by both of the firstand second pacing pulses, capture by a pacing pulse delivered only fromthe first pacing channel, and capture by a pacing pulse delivered onlyfrom the second pacing channel; means for determining whether or noteach of the first and second pacing pulses have achieved capture inaccordance with which of the template electrograms is most similar tothe test electrogram.
 17. The system of claim 16 wherein the means fordelivering first and second pacing pulses deliver pacing pulses toeither the right and left ventricles or the right and left atria. 18.The system of claim 16 further comprising means for varying the pulseenergy of the pacing pulses in order to determine a capture thresholdfor a pacing channel.
 19. The system of claim 18 further comprisingmeans for determining a capture threshold for each of the first andsecond pacing channels by lowering the pacing energy of each pacingchannel separately until the test electrogram no longer matches thetemplate electrogram representing capture by both of the first andsecond pacing pulses.
 20. The system of claim 18 further comprisingmeans for determining a capture threshold of the first and second pacingchannels by: lowering the pacing energy of the first pacing channeluntil the test electrogram matches the template electrogram representingcapture by the second pacing pulse but does not match the templateelectrogram representing capture by the first pacing pulse; and,lowering the pacing energy of the second pacing channel until the testelectrogram matches a template electrogram representing capture by thefirst pacing pulse but does not match the template electrogramrepresenting capture by the second pacing pulse.